Tear the bread into a large mixing bowl and add the fruit, peel and spice. Pour in the milk, then stir or scrunch through your fingers to mix everything well and completely break up the bread. Add eggs, muscovado and lemon zest if using. Stir well, then set aside for 15 mins to soak.
Heat oven to 180C/160C fan/gas 4. Butter and line the base of a 20cm non-stick square cake tin (not one with a loose base). Stir the melted butter into the pudding mixture, tip into the tin, then scatter with demerara. Bake for 1½ hrs until firm and golden, covering with foil if it starts to brown too much. Turn out of the tin and strip off the paper. Cut into squares and serve warm.